Doherty amplifier

ABSTRACT

The present invention is intended to provide a Doherty amplifier that can accomplish a linear amplifying and a power combining operation closer to the ideal in a simple configuration. The configuration comprises gain compensator  6  that is composed of a parallel circuit made of an anti-parallel diode and a resistor that is disposed at a position ahead of peak amplifier  4  included in the Doherty amplifier. Setting the gain in gain compensator  6  allows peak amplifier  4  to be compensated for the operation characteristic, when peak amplifier  4  is operating, based on the gm characteristic of peak amplifier  4.

TECHNICAL FIELD

The present invention relates to a Doherty amplifier, and moreparticularly, to a Doherty amplifier, which has a gain compensator thatis disposed at a position ahead of a peak amplifier.

BACKGROUND ART

Due to the explosive proliferation of the portable terminal market inrecent years and improvements in the infrastructure associatedtherewith, stricter requirements have been made from the market forimprovements in the efficiency of transmission amplifiers for basestations.

In order to respond to the foregoing requirements, attention has beenfocused in recent years on the trend of attempts to buildhigh-performance and highly efficient amplifiers by combining technologyto amplify signals at high efficiency, as represented by the Dohertyamplifier, with technology to reduce distortions therefor together withrecent distortion compensation technology.

The Doherty amplifier is a device for improving the efficiency of ahigh-output power amplifier, which was first proposed in Document 1 (W.H. Doherty “A New High Efficiency Power Amplifier for Modulated Waves”,Proc.

IRE, Vol. 24, No. 9, Sept. in 1936).

The Doherty amplifier comprises a carrier amplifier for performing anamplifying operation at all times; and a peak amplifier for performingan amplifying operation when high power is generated, specifically, onlyafter the carrier amplifier has reached a saturated maximum output.

In the Doherty amplifier, devices having the same characteristics aregenerally used for the carrier amplifier and peak amplifier, and theyare arranged in parallel. A large number of Doherty amplifiers have beenactually implemented as amplifiers to handle signals in frequency bandsfrom low frequencies to millimeter waves.

The example described in Document 2 (JP-7-22852-A) is an example of sucha kind of Doherty amplifiers that is conventionally used. FIG. 1illustrates the Doherty amplifier described in Document 2. In thefollowing, the Doherty amplifier described in Document 2 will bedescribed in brief with reference to FIG. 1.

In FIG. 1, a signal applied from input terminal 1 is distributed to aside of the carrier amplifier and to a side of the peak amplifier byinput branching circuit 2, which includes one-quarter wavelengthtransmission path 21. Carrier amplifier 3 amplifies a signal that isdistributed to the carrier amplifier side. A signal distributed to thepeak amplifier side is amplified by peak amplifier 4 after it has passedthrough one-quarter wavelength transmission path 21 f.

Output combiner circuit 5 includes one-quarter wavelength transmissionpath 51. Output combiner circuit 5 combines the output of carrieramplifier 3, which has passed one-quarter wavelength transmission path51, with the output of peak amplifier 4 to deliver the resulting output.Therefore, a phase relationship between output signals of carrieramplifier 3 and peak amplifier 4 is identical at the signal combiningpoint of output combiner circuit 5.

However, if an amplifying operation of carrier amplifier 3 or peakamplifier 4 of the Doherty amplifier differs from an ideal operation,the signal combination, which is performed by output combiner circuit 5,is not performed in an effective manner. For this reason, the Dohertyamplifier fails to provide an ideal linear amplifying action andsaturated output power.

For example, the foregoing problem occurs when devices having equivalentcharacteristics (for example, the gm-Id characteristic) are used for acarrier amplifier and a peak amplifier, which make up the Dohertyamplifier (classical Doherty). In this event, a problem arises,particularly, in that the operation of the peak amplifier differs fromoptimum performance. Specifically, a problem occurs in that the gain inthe peak amplifier is smaller than a optimum gain.

Therefore, an ideal linear amplifying action or saturated output powercannot be provided even if the carrier amplifier and peak amplifier areidentical in the gm (transfer conductance) characteristic (FET's or thelike).

Several approaches for addressing this problem have been proposed.

For example, Document 3 (RF Power Amplifiers for Wirelesscommunications, written by Steve C. Cripps, p236, Artech House, 1999)has proposed a technique for controlling the attenuation amount of avariable attenuator, which is disposed on the input side of a peakamplifier, in accordance with the magnitude of the input level in orderto compensate for the transfer characteristic.

Also, Document 4 (Advanced Techniques in RF Power Amplifiers written bySteve C. Cripps, P50, Artech House, 2002) has proposed a method ofgenerating maximum power of a Doherty amplifier by appropriatelycontrolling the bias setting of a carrier amplifier from a class-C biasto a class-B bias, in accordance with an input signal level, though nospecific block diagram or the like is found therein.

Further, Document 5 (Published Japanese Translation of PCT InternationalPublication for Patent Application No. 2000-513535) has proposedtechniques by which a detector directly or indirectly detects the powerlevel of an input signal and the magnitude of the signal, such that biascontrollers of a carrier amplifier and a bias amplifier control biasesfor the carrier amplifier and peak amplifier, respectively, relying onthe detected value.

However, whether it be the techniques of Documents 3, 4 or 5, they allrequires circuits for making determinations, control and the like, thusleading to a problem that the configuration becomes complicated.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a Doherty amplifierwhich is capable of accomplishing a linear amplifying and a powercombining operation closer to the ideal in a simple configuration, forexample, even if the devices with the same characteristics are used as acarrier amplifier and a peak amplifier.

To achieve the above object, a Doherty amplifier according to thepresent invention has an input terminal, input branching means fordistributing the signal applied from the input terminal to a first pathand a second path, a carrier amplifier for amplifying the signaldistributed to the first path by the input branching means, a peakamplifier for amplifying the signal of a predetermined level or higheramong the signals distributed to the second path by the input branchingmeans, output combining means for combining the output of the carrieramplifier with the output of the peak amplifier, and a gain compensator,which is disposed at a position ahead of the peak amplifier in thesecond path, for changing a gain in accordance with the level of aninputted signal in order to correct the level of the signal distributedto the second path.

The invention described above includes the gain compensator, which isdisposed at the position ahead of the peak amplifier in the second path,for changing a gain in accordance with the level of an input signal inorder to correct the level of the signal distributed to the second path.Thus, it is possible to compensate for the gain of the peak amplifier bya gain varying operation of the gain compensator. It is thereforepossible that circuits, which have been conventionally required forperforming detection, determination, control and the like, areunnecessary. Consequently, it is possible to carry out a linearamplifying and a power combining operation closer to the ideal in asimple configuration. Since it is possible that a control circuit, whichhas been conventionally required, is unnecessary, it is also possiblethat dedicated control signals and dedicated terminals for the controlsignals are unnecessary.

Also, in the Doherty amplifier according to the present invention, again in the gain compensator, when a signal lower than the predeterminedlevel is applied, is different from a gain in the gain compensator whena signal equal to or higher than the predetermined level is applied.

According to the invention described above, in the gain compensator, thegain, when a signal lower than the predetermined level is applied, isdifferent from the gain when a signal equal to or higher than thepredetermined level is applied. Thus, in addition to the aforementionedeffects, the peak amplifier can be compensated for the gain when thepeak amplifier performs an amplifying operation, without changing theamplifying operation start point of the peak amplifier for a signalapplied from the input terminal.

Also, in the gain compensator of the Doherty amplifier according to thepresent invention, the gain, when the signal equal to or higher than thepredetermined level is applied, is larger than the gain when the signallower than the predetermined level is applied.

According to the present invention described above, in addition to theaforementioned effects, the gain of the peak amplifier can be improvedwhen the peak amplifier performs an amplifying operation, withoutchanging the amplifying operation start point of the peak amplifier fora signal applied from the input terminal. Consequently, when the gain inthe peak amplifier is smaller than an optimum gain, the gain in the peakamplifier can be compensated for when the peak amplifier performs anamplifying operation, without changing the amplifying operation startpoint of the peak amplifier.

Also, in the gain compensator of the Doherty amplifier according to thepresent invention, the gain, when the signal equal to or higher than thepredetermined level is applied, is smaller than the gain when the signallower than the predetermined level is applied.

According to the invention described above, in addition to theaforementioned effects, the gain of the peak amplifier can be reducedwhen the peak amplifier performs an amplifying operation, withoutchanging the amplifying operation start point of the peak amplifier fora signal applied from the input terminal. Consequently, when the gain inthe peak amplifier is larger than the ideal gain, the gain in the peakamplifier can be compensated for when the peak amplifier performs anamplifying operation, without changing the amplifying operation startpoint of the peak amplifier.

Also, in the Doherty amplifier according to the present invention, thegain of the gain compensator is set based on the operationcharacteristic of the peak amplifier. Thus, in addition to theaforementioned effects, the gain in the peak amplifier can becompensated for with high accuracy.

Also, in the Doherty amplifier according to the present invention, thegain compensator is a parallel circuit of an anti-parallel diode and aresistor, or a parallel circuit of a diode and a resistor, or an FET, ora bipolar transistor. Thus, in addition to the aforementioned effects, asimple configuration can be used to implement the gain compensator.

Also, in the Doherty amplifier according to the present invention, thecarrier amplifier and peak amplifier are each composed of an FET, andthe gain compensator compensates for the gm characteristic of the peakamplifier.

According to the invention described above, even when the carrieramplifier and peak amplifier are composed of FET's, it is possible toprovide effects similar to the aforementioned effects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a conventional Doherty amplifier;

FIG. 2 is a block circuit diagram illustrating one embodiment of thepresent invention;

FIG. 3 a is a circuit diagram illustrating an example of gaincompensator 6;

FIG. 3 b is an explanatory diagram showing a characteristic of gaincompensator 6;

FIG. 4 is an explanatory diagram for describing operating states ofamplifiers 3 and 4 during an ideal operation of the Doherty amplifier;

FIG. 5 is a characteristic diagram showing the drain current—gatevoltage characteristics of carrier amplifier 3 and peak amplifier 4;

FIG. 6 is an explanatory diagram illustrating an operating state of theDoherty amplifier;

FIG. 7 is a characteristic diagram showing the characteristic of thegain compensator illustrated in FIG. 3 a;

FIG. 8 is an explanatory diagram showing an example of compensation bythe gain compensator in accordance with the peak amplifier;

FIG. 9 a is a circuit diagram illustrating another example of the gaincompensator;

FIG. 9 b is a circuit diagram illustrating another example of the gaincompensator; and

FIG. 9 c is a circuit diagram illustrating another example of the gaincompensator.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, embodiments of the present invention will be describedwith reference to the drawings.

One feature of this embodiment is that gain compensator 6 is disposed ata position ahead of peak amplifier 4, as illustrated in FIG. 2, in aDoherty amplifier that includes carrier amplifier 3 and peak amplifier4. In FIG. 2, a gain compensator, as a correcting means, has a gain thatvaries in accordance with an input level. Note that in FIG. 2, thoseidentical in configuration to those illustrated in FIG. 1 are designatedwith the same reference numerals.

This embodiment will be summarized here.

In the Doherty amplifier of this embodiment, gain compensator 6, whosegain varies in accordance with an input level, is disposed in aconventional configuration which comprises carrier amplifier 3, peakamplifier 4, output combiner 5, and input branching circuit 2, asillustrated in FIG. 1. Specifically, as illustrated in FIG. 2, gaincompensator 6 is disposed at a position ahead of peak amplifier 4.

In this embodiment, when the level of an input signal is equal to orhigher than an amplification start level of peak amplifier 4 which isbiased, for example, to class C and composed of FET, gain compensator 6makes a correction such that it increases the gate input voltage signalof peak amplifier 4.

By using gain compensator 6 as described above, even in a Dohertyamplifier has peak amplifier 4 and carrier amplifier 3 that have thesame characteristics, the Doherty amplifier can operate to generate adesired maximum output at the time of a saturated output. It istherefore possible to achieve optimum amplifying performance as aDoherty amplifier.

In the following, the Doherty amplifier of this embodiment will bedescribed specifically with reference to FIG. 2.

This Doherty amplifier has amplifier 3, generally called a “carrieramplifier,” which amplifies a signal at all times; and amplifier 4,generally called a “peak amplifier” or an “auxiliary amplifier (in thisspecification, the “peak amplifier” is always used), which operates onlywhen a signal having a predetermined level or higher is applied (onlywhen high power is generated).

The predetermined level corresponds to the level of a signal that isapplied from input terminal 1 when carrier amplifier 3 starts generatingsaturated output power. In this embodiment, the predetermined level isset at the level of a signal applied from input terminal 1 when carrieramplifier 3 starts generating the saturated output power.

Gain compensator 6 is disposed at a position ahead of peak amplifier 4to compensate for an amplitude component of peak amplifier 4 inaccordance with the transfer characteristic (operation characteristic)of peak amplifier 4.

Further, this Doherty amplifier includes output combiner circuit 5 as anoutput combining means for combining the output of carrier amplifier 3with the output of peak amplifier 4 in order to deliver the combinedoutput; and input branching circuit 2 as an input branching means fordistributing an input signal to carrier amplifier 22 (first path) andthe side of peak amplifier 23 (second path).

Generally, a Doherty amplifier has carrier amplifier 3 that operateswhile maintaining saturation near the saturated output power. Thus, theDoherty amplifier accomplishes a higher efficiency than general class-Aand class-AB amplifiers even when a backoff is removed from thesaturated power for delivery.

Generally, an amplifier, which is biased to class AB or class B, isoften used as carrier amplifier 3. Generally, peak amplifier 4 is oftenbiased to class C for use such that it operates only when a high-powersignal is generated.

Output combiner circuit 5, which combines the output of carrieramplifier 3 with the output of peak amplifier 4, is composed, forexample, of a transformer, and generally comprises one-quarterwavelength transmission line 51. Input branching circuit 2 includesone-quarter wavelength transmission line 21 or a 90° hybrid circuit orthe like for making a wave relationship between an output signal of peakamplifier 4 and an output signal of carrier amplifier 3 that isidentical in pattern at a signal combining point of output combinercircuit 5.

Also, gain compensator 6 of FIG. 2 is composed of anti-parallel diode 61and resistor 62, for example, as illustrated in FIG. 3 a.

Specifically, a parallel circuit of anti-parallel diode 61 and resistor62 may be used as gain compensator 6. Anti-parallel diode 61 is composedof diode 61 a and diode 61 b. Diode 61 a has a cathode that is connectedto one-quarter wavelength transmission line 21 of input branchingcircuit 2, and a cathode that is connected to the input side of peakamplifier 4. Diode 61 b has a cathode that is connected to the anode ofdiode 61 a, and an anode that is connected to the cathode of diode 61 a.

Because the operation principles of the general Doherty amplifier arewell known by those skilled in the art, for example, from documents suchas Advanced Techniques in RF Power Amplifiers, Artech House 2002 writtenby Steve C. Cripps, detailed description thereon is omitted.

In the following, the operation of this embodiment will be described.

For simplifying the description, this embodiment employs carrieramplifier 3, which is biased to class B, and peak amplifier 4, which isbiased to class C, and employs FET devices of the same characteristicsas carrier amplifier 3 and peak amplifier 4. The operation of thisDoherty amplifier will be described below. However, the presentinvention is not limited to the foregoing configuration, but can bemodified as appropriate.

At first, the operating states of respective amplifiers 3, 4, in whichthe Doherty amplifier achieves optimum performance, will be describedwith reference to FIG. 4.

The operation of the Doherty amplifier is roughly divided into threeoperation regions, specifically, a low level region, a transitionregion, and a saturation region.

In FIG. 4, the horizontal axis represents input voltage Vin, whichindicates a maximum value of an input voltage, which is applied to eachof carrier amplifier 3 and peak amplifier 4, as 1, and the vertical axisrepresents a drain current of peak amplifier 4 as Ip, an output voltageof carrier amplifier 3 as Vc, and a drain current of carrier amplifier 3as Ic.

In this embodiment, assume that FET's of the same characteristics areused as carrier amplifier 3 and peak amplifier 4. For this reason, amaximum value of Ic and a maximum value of Ip are shown to be equal.Also, peak amplifier 4, which is a component of a normal Dohertyamplifier, is biased to class C, and starts an amplifying operation onlyafter the drain current starts flowing from input voltage Vin that is0.5.

Also, for the drain current gate voltage characteristics of therespective devices, specifically, carrier amplifier 3 and peak amplifier4, assume that the drain current starts flowing from threshold voltageVth, as shown in FIG. 5, and transfer conductance gm has a constantvalue.

When signal Vin at a level equal to or lower than a predetermined level(Vin=0.5 in this example) is applied to carrier amplifier 3 that isbiased to class B, output voltage Vc of carrier amplifier 3 is generatedin proportion to signal Vin. In this event, a region over which outputvoltage Vc of carrier amplifier 3 varies in accordance with signal Vin,defines the low level region.

Next, when signal Vin reaches 0.5 (this is called the “transitionpoint”), carrier amplifier 3 is saturated, so that the output voltagebecomes a constant value. At this time, the efficiency of the Dohertyamplifier itself is maximized, where the efficiency ideally reaches 78%(π/4) which is an ideal efficiency of a class-B amplifier. However, thesaturated output power of carrier amplifier 3 at this time is onequarter of the saturated power that should be generated as the Dohertyamplifier.

When Vin increases from this transition point, peak amplifier 4 alsostarts operating.

With this operation of peak amplifier 4, a load impedance of carrieramplifier 3 modulates through transmission transformer 51 of outputcombiner circuit 5. As a result, the output current of carrier amplifier3 increases linearly in accordance with the input voltage, so that aload is supplied with larger power. As a result, in the Dohertyamplifier, a linear amplification characteristic is maintained.Consequently, the Doherty amplifier can linearly amplify the power.

When the input voltage further increases, peak amplifier 4 also reachessaturation. As a result, a saturated maximum output is generated as theDoherty amplifier. For the time period from this transition point to asaturation point, the total efficiency of the Doherty amplifier ismaintained extremely high.

The foregoing operation is an example of an ideal operation of theDoherty amplifier.

In this event, the drain current of peak amplifier 4 must increase inproportion to the input voltage, which is higher than the transitionpoint, with a slope twice as large as the slop of an increase in thedrain current of carrier amplifier 3. In this event, when the inputvoltage finally reaches a peak (Vin=1.0), the drain current (Ic) ofcarrier amplifier 3 and the drain current (Ip) of peak amplifier 4 aremaximized (Ic=Ip=1.0). Consequently, the load impedance, which is viewedfrom carrier amplifier 3, is also in an optimal state in which themaximum output can be transferred to the load, so that the maximumoutput of the Doherty amplifier can be generated.

Next, a description will be given of an actual operation of the Dohertyamplifier.

When the Doherty amplifier is actually designed, devices havingsubstantially the same characteristics are often used for carrieramplifier 3 and peak amplifier 4. This is a configuration called“classical Doherty.”

For example, when the saturated power of a Doherty amplifier is set at100 W, the devices with the same characteristics, the saturated power ofwhich is 50 W, are used as a carrier amplifier and a peak amplifier. Ofcourse, instead of this, a configuration called “extended Doherty” maybe used, in which the devices differ in saturated power. However,because the basic principles are the same, description and the like forthat case are omitted.

However, when a Doherty amplifier is made up by using devices which havethe same characteristics as a carrier amplifier and a peak amplifier asdescribed above, ideal characteristics of the Doherty amplifier cannotbe accomplished just by simply combining the carrier amplifier with thepeak amplifier, having the same characteristics, as illustrated in FIG.1 as the prior art. For this reason, a lower coefficient, lowersaturated power, and degraded linearity will result near the saturatedpower.

FIG. 6 is a diagram showing an example of the degradation which occursin an actual Doherty amplifier, showing the input/output characteristicsof main parameters similar to the ideal state shown in FIG. 4.

As described above, in the ideal state, the current of peak amplifier 4must reach the maximum value at the maximum point of the input voltage.Contrary to this, in the example shown in FIG. 6, transfer conductancegm is only one-half of a value required as an ideal value. Thus, even ifthe input voltage is maximized, the drain current merely reachesone-half of an ideal value.

For this reason, an ideal operation of the Doherty amplifier fails.According to a simple calculation, the drain efficiency at the maximuminput is 58.9%, which is about 20% lower than 78% that occurs in theideal state, the output decreases to 50% of the ideal state, and theinput/output linearity degrades to the output level that is 0.5 when theinput is 1.

Thus, in the present invention, a Doherty amplifier that operates atoptimum performance can be implemented by disposing a gain compensator,the gain of which varies in accordance with the magnitude of an inputsignal, as an example illustrated in FIG. 3 a, at a position ahead ofpeak amplifier 4. For example, even if devices that have the samecharacteristics are used as carrier amplifier 3 and peak amplifier 4,the resulting product can operate as an ideal Doherty amplifier.

In the case of this embodiment, specifically, the characteristics ofgain compensator 6 shown in FIG. 3 b may be set such that the outputsignal increases substantially by a factor of two in response to anincrease by one of the input signal when signal Vin is at 0.5 or more,as shown in FIG. 7.

The characteristics as mentioned above can be approximately accomplishedby selecting appropriate diodes 61 a, 61 b and peripheral resistor 62 inthe exemplary circuit as illustrated in FIG. 3 a. For example, as alarge resistance value is selected, an output range characteristic isshown in which the input/output characteristic with a large slopeoccupies a larger proportion, and conversely, as a small resistancevalue is selected, an output range characteristic is shown in which theinput/output characteristic with a large slope occupies a smallerproportion.

Consequently, when this gain compensator 6 is disposed at a positionahead of peak amplifier 4, and when the input level to gain compensator6 and an operating state are set such that the output range start pointof gain compensator 6 is positioned near the threshold voltage (Vin=0.5)of peak amplifier 4 or at the threshold voltage (Vin=0.5) of peakamplifier 4, the gm characteristic of peak amplifier 4 can be apparentlydoubled, making use of a region in which the slope of the input/outputcharacteristic is approximately two.

Stated another way, peak amplifier 4 has a maximum drain current at thetime that the input level reaches the maximum value. Therefore, it ispossible to accomplish an ideal state of the Doherty amplifier from thetransition point to the saturated state. As such, even if the deviceswith the same characteristics, for example, are used for the carrieramplifier and peak amplifier, it is possible to implement a Dohertyamplifier that can present a linear amplifying and a power combiningoperation closer to the ideal in a simple configuration.

Describing in greater detail, in the case of this example, the inputlevel to or the operating state of gain compensator 6 is set such thatthe gain of gain compensator 6, i.e., the slope of the input/outputcharacteristic of gain compensator 6 is one or substantially one whenthe level of a signal applied to input terminal 1 is equal to or lowerthan the threshold voltage of peak amplifier 4, and the gain of gaincompensator 6, i.e., the slope of the input/output characteristic ofgain compensator 6 is two or substantially two when the level of thesignal applied to input terminal 1 exceeds the threshold voltage of peakamplifier 4. It is therefore possible to operate peak amplifier 4 in anideal state.

As described above, gain compensator 6, the gain of which varies inaccordance with the level of an input signal, is disposed at theposition ahead of peak amplifier 4. Thus, it is possible to compensatefor the gain during the operation of the peak amplifier by a gainvarying operation of gain compensator 6. Also, it is possible to avoidan unintended operation of peak amplifier 4 when there is no need forthe peak amplifier to operate (in a state where the level of a signal,which is applied to input terminal 1, is below the predetermined level).

Supplementing this aspect, assume, for example, that the gain in gaincompensator 6 is fixed at a gain to compensate for peak amplifier 4during its operation, then peak amplifier 4 can unexpectedly operateeven in a state where peak amplifier 4 should essentially not operate(in the state where the level of a signal applied to input terminal 1 isbelow the predetermined level).

On the contrary, in this embodiment, gain compensator 6, the gain ofwhich varies in accordance with the level of an input signal, isdisposed at a position ahead of peak amplifier 4. Thus, the gain of gaincompensator 6 can be modified such that this gain goes to a gain atwhich the output of gain compensator 6 is below the predetermined valueat which peak amplifier 4 should essentially not operate (in the statewhere the level of a signal applied to input terminal 1 is below thepredetermined level), and the gain of gain compensator 6 provides a gainwhich compensates the peak amplifier in the state where the level of asignal applied to input terminal 1 is the same or higher than thepredetermined level. It is therefore possible to perform a linearamplifying and a power combining operation closer to the ideal in asimple configuration as described above.

Also, because the gain of gain compensator 6, when a signal that islower than the predetermined level is applied from input terminal 1, isset different from the gain of gain compensator 6 when a signal that isequal to or higher than the predetermined level is applied, the linearamplifying and power combining operations closer to the ideal can beperformed in a simple configuration, as described above, withoutchanging the amplifying operation start point of peak amplifier 4 (thethreshold voltage of peak amplifier 4) for the signal applied from inputterminal 1.

Also, when the gain of gain compensator 6, when a signal equal to orhigher than the predetermined level, is larger than the gain of gaincompensator 6 when a signal lower than the predetermined level isapplied, the gain of the peak amplifier, when the peak amplifierperforms an amplifying operation, can be increased to an ideal gainwithout changing the amplifying operation start point of the peakamplifier for the signal applied from the input terminal, if the gain ofpeak amplifier 4 is below the ideal value.

As well, the gain of gain compensator 6 is set based on the operationcharacteristic of peak amplifier 4. Thus, in addition to theaforementioned effects, the gain of peak amplifier 4 and/or theamplifying operation start point of peak amplifier 4 (the thresholdvoltage in this example) can be compensated for with high accuracy.

In the foregoing, an example has been shown for the case where transferconductance gm of peak amplifier 4 is just one-half as much as a valuethat would be required as an ideal value, and an example has been shownfor the case where the slope of the input/output characteristic of gaincompensator 6 is set to two or approximately two as the gaincompensation of gain compensator 6, when signal Vin is equal to or morethan 0.5. However, the gain compensation of gain compensator 6 whensignal Vin is equal to or more than 0.5 can be changed as appropriatedepending on the ratio of transfer conductance gm of peak amplifier 4 tothe ideal value.

For example, when transfer conductance gm of peak amplifier 4 is largerthan the value required as the ideal value, the slope of theinput/output characteristic of gain compensator 6 may be set to one orapproximately one, as the gain compensation of gain compensator 6, ifsignal Vin is less than 0.5. On the other hand, if signal Vin is equalto or more than 0.5, the slope of the input/output characteristic ofgain compensator 6 may be reduced to less than one, as the gaincompensation of gain compensator 6.

As described above, when using gain compensator 6, whose gain issmaller, when a signal equal to or higher than a predetermined level isapplied, than the gain, when a signal lower than the predetermined levelis applied, the gain of the peak amplifier can be reduced when the peakamplifier performs an amplifying operation, without changing theamplifying operation start point of the peak amplifier to the signalthat is applied from the input terminal. Thus, when the gain of the peakamplifier is larger than the ideal gain, the peak amplifier can becompensated for this gain when the peak amplifier performs an amplifyingoperation, without changing the amplifying operation start point of thepeak amplifier.

Also, the gain compensator is not limited to the configurationillustrated in FIG. 3 a, but can be modified as appropriate. Forexample, gain compensator 6 can also be implemented by a simple circuitas illustrated in FIG. 9 which has characteristics as described above.

FIG. 9 will be described in brief. FIG. 9 a is a diagram illustrating anexample in which a parallel circuit of diode 63 and resistor 64 are usedas gain compensator 6. In FIG. 9 a, diode 63 has a cathode that isconnected to one-quarter wavelength transmission line 21 of inputbranching circuit 2, and diode 63 has an anode that is connected to theinput side of peak amplifier 4. FIG. 9 b is a diagram illustrating anexample in which FET 65 is used as gain compensator 6. In FIG. 9 b, FET65 has a drain that is connected to second path 23, and the FET has agrounded source. FIG. 9 c illustrates an example in which bipolartransistor 66 is used as gain compensator 6. In FIG. 9 c, bipolartransistor 66 has a collector that is connected to second path 23,bipolar transistor 66 has a grounded emitter, and desired voltage VB isapplied to the base of bipolar transistor 66.

Also, the concept of the present invention can be readily extended toamplifiers having different saturated current characteristics of devicesthat are used for peak amplifier 4 and carrier amplifier 3. For example,even when devices, which differ in source voltage or saturated currentcharacteristic, are used as peak amplifier 4 and carrier amplifier 3, anapproach substantially identical to the foregoing can be basicallyemployed, by taking into consideration of the input/outputcharacteristic which is normalized by a maximum value, as describedabove. To give a specific illustration, as shown in FIG. 8, a slope ofthe peak amplifier, from a class-C bias point to an operating currentvalue, in which the desired saturated output power can be generated, maybe corrected by the gain compensator using a desired value. Inparticular, a slope of the peak amplifier, from its amplifying operationstart point (for example, the threshold voltage) to the operatingcurrent value in which the desired saturated output power can begenerated, for example, the gm characteristic, may be corrected by thegain compensator using a desired value.

Therefore, even when devices, which have different characteristics fromeach other, are used as carrier amplifier 3 and peak amplifier 4, it ispossible to implement a Doherty amplifier which provides a linearamplifying and a power combining operation close to the ideal in asimple configuration.

In the present invention, the gain compensator, whose gain varies inaccordance with the level of an input signal, is disposed at a positionahead of the peak amplifier of the Doherty amplifier. It is thereforepossible to perform an ideal amplifying operation at optimum performancewithout requiring a complicated circuit configuration and control forperforming detection, determination, control and the like, as is thecase with the prior art. Consequently, it is possible to simplify theconfiguration and reduce the cost even when the devices with the samecharacteristics, for example, are used as the carrier amplifier and peakamplifier.

In the embodiment described above, the illustrated configurations aremere examples, and the present invention is not limited to theconfigurations.

1. canceled
 2. canceled
 3. A Doherty amplifier comprising: an inputterminal; input branching means for distributing a signal applied fromsaid input terminal to a first path and a second path; a carrieramplifier for amplifying a signal distributed to the first path by saidinput branching means; a peak amplifier for amplifying a signal of apredetermined level or higher among signals distributed to the secondpath by said input branching means; output combining means for combiningan output of said carrier amplifier with an output of said peakamplifier; and a gain compensator disposed at a position ahead of saidpeak amplifier in the second path for changing a gain in accordance withthe level of an input signal in order to correct the level of the signaldistributed to the second path, said carrier amplifier and said peakamplifier being devices having the same characteristics, said peakamplifier having a gain smaller than an ideal gain, wherein said gaincompensator has a larger gain, when a signal equal to or higher than thepredetermined level is applied, than a gain when a signal lower than thepredetermined level is applied, said gain being set based on a transferconductance of said peak amplifier.
 4. canceled
 5. canceled
 6. TheDoherty amplifier according to claim 3, wherein said gain compensatorcomprises a parallel circuit composed of an anti-parallel diode and aresistor, or a parallel circuit composed of a diode and a resistor, oran FET, or a bipolar transistor.
 7. The Doherty amplifier according toclaim 3, wherein said carrier amplifier and said peak amplifier are eachcomposed of an FET, and said gain compensator compensates said peakamplifier for a gm characteristic.